1. Field of the Invention
The invention relates to methods for inducing superplastic deformation in a composite. More particularly, the invention relates to cycling a composite material including a transforming phase through a phase transformation of the transforming phase while applying an external stress to the composite material to induce superplastic deformation.
2. Description of the Prior Art
The phenomenon of superplastic deformation (i.e. deformation of a material at low stresses and to very large strains before failure) is known to exist in bulk metals and ceramics. The term "bulk" is used to refer to a non-composite material which is single phase on a macroscopic scale.
Superplastic deformation of such bulk materials can result from internal stress caused by anisotropy in the thermal expansion coefficient of the material as described by Wu et al., "Internal Stress Superplasticity in Anisotropic Polycrystalline Zinc and Uranium", Metallurgical Transactions A, 18A, 451-462 (1987). Bulk material superplastic deformation behavior can also be induced by a phase transformation of the material as reported by de Jong et al., "Mechanical Properties of Iron and Some Iron Alloys While Undergoing Allotropic Transformation", Acta Metallurgica, 7, 246-253 (1959) for bulk iron and iron alloy metals. Also, superplastic deformation behavior in a bulk metal can be the result of the particular grain structure characteristic of the metal such as the fine grain-induced superplasticity of bulk titanium metal described in U.S. Pat. No. 4,263,375, to Elrod, issued Apr. 21, 1981.
In composite materials, where at least two distinct phases are macroscopically identifiable, superplastic deformation behavior resulting from thermal cycling has been observed and is attributed to the difference between the coefficients of thermal expansion of the matrix phase material and of the reinforcement phase material as described, for example, by Pickard et al., "The Deformation of Particle Reinforced Metal Matrix Composites During Temperature Cycling", Acta metall. mater., 38, 2537-2552 (1990).
Metal matrix composites of particular interest for many industrial applications are titanium metal matrix composites. Titanium is prized for its specific strength and specific stiffness at both ambient and elevated temperatures. However, titanium lacks the stiffness needed for some aerospace applications. Adding ceramic particles or fibers to a titanium matrix increases the strength-to-weight and stiffness-to-weight ratios. However, titanium and its alloys are difficult to work because of their resistance to deformation at the optimum hot-working temperature. Titanium matrix composites are even more difficult to form and machine.
Currently, superplastic forming of titanium is accomplished by using the phenomenon of fine grained superplasticity. This type of superplasticity is limited by restrictions on the temperature range, strain rate, .epsilon., and grain size. Fine grained superplastic forming can only be accomplished at small strain rates. This limits the rate at which titanium parts can be produced. Another problem with superplastic forming is the requirement that a small grain size be maintained throughout the superplastic deformation.
Pure titanium is characterized by an allotropic metal phase transformation: below 882.degree. C., its structure is hexagonal-close-packed (hcp, .alpha. phase). Above 882.degree. C., the hcp structure transforms to body centered cubic (bcc, .beta. phase) which is mechanically weaker than the .alpha. phase. Weakening of mechanical properties also occurs during the .alpha.77 .fwdarw..beta. transformation. This weakening manifests itself by:
1. an enhanced creep rate for deformation at constant stress, or PA1 2. a stress drop for constant strain-rate tests
and is referred to as transformation plasticity. This phenomenon results from an interaction between the internal stresses from the phase transformation and the macroscopic stresses from the external load.
Thus, there exists a need for a method for inducing superplasticity in a composite and for forming a part from a composite material including a phase which undergoes a phase transformation which allows for the forming of composites which are typically difficult to form, and likely to fail at low strains, and which provides enhanced strain per cycle, resulting in faster composite deformation and, thus, less time to form a part. There exists a particular need for such an efficient forming process for industrially important titanium/titanium carbide composites. A low-to-moderate temperature method to form these titanium composites could reduce the cost of shaping the high performance parts needed by many industries.